1. Field of the Invention
This invention provides a new apparatus and process for use in preparing single crystals of alkaline earth fluorides from the melt which reduces the processing time and increases the purity of the resultant crystals while conserving reaction materials and energy.
2. Description of the Prior Art
In the prior art, single crystals of alkaline earth halides, rare earth halides and alkali metal halides have been grown from melts of selected reactants by crystal growth methods commonly known as the Czochralski-, Bridgman-Stockbarger-, or Storber-method or by various combinations thereof. These processes, and the apparatus intended to facilitate the preparation of crystals by the processes, are illustrated in U.S. Pat. Nos. 1,793,672; 3,139,653 and 4,242,589, all of which describe methods of forming crystals from a melt of various reactants under a static atmosphere.
The growth of alkaline earth chloride, bromide and iodide single crystals from a melt under a dynamic reactive atmosphere (RA) is disclosed in U.S. Pat. No. 4,190,487 issued to Ricardo C. Pastor et al in February 1980. The growth of alkali metal chloride bromide and iodide single crystals from a melt under a dynamic reactive atmosphere is disclosed in U.S. Pat. No. 4,076,574 issued to Ricardo C. Pastor et al in February 1978.
The use of a RA of hydrogen fluoride (HF) above the melt, during the crystal growth process to reduce contamination of fluoride crystals, is disclosed in: U.S. Pat. No. 3,649,552 issued to Morton Robinson et al for a "Method for Preparing High Quality Rare Earth and Alkaline Earth Fluoride Single Crystals" in March 1972; in U.S. Pat. No. 3,769,230 issued to Morton Robinson et al for "Calcium Fluoride Rare Earth Fluoride Fluorescent Compound Used as a Laser Crystal" in October 1973; and in U.S. Pat. No. 3,935,302 issued to Ricardo C. Pastor et al for "Preparation of Alkaline Earth Metal Halide Crystals for Laser Windows" in January 1976.
Each of the above-mentioned prior art devices and processes share one common limitation in that the crystals are grown in a crucible wherein the melt, and crystal subsequently grown therefrom, are exposed to the atmosphere existing within the furnace chamber. This defect renders it difficult, if not impossible, to remove all traces of anion and/or cation contaminants from the crystal growth atmosphere and thus from the crystals grown therein. The importance of removing OH.sup.- and O.sup.-- contaminants from the crystal growth atmosphere of halide crystals is discussed at length in: U.S. Pat. Nos. 3,649,552; 3,769,230; 3,935,302; 3,959,442; 4,076,574; and 4,190,487. These teachings are incorporated herein by reference. However, notwithstanding the recognition of this importance by others, each of the prior art devices and processes which utilized the devices are limited in their ability to meet this need by the inherent outgassing characteristics of their respective crystal growth furnace. All of the prior art crystal growth furnaces comprised heating elements mounted within a chamber with baffles, shields and insulation. These elements were, and are, an essentially infinite source of contaminants. Before initiating a crystal growth process via prior art methods, the furnace chambers were either evacuated, flushed with an inert gas, filled with a reactive gas or flushed with a reactive gas as the temperature was raised to cause the reactants to melt. In the RA process disclosed by Pastor et al, the furnaces are vacuum baked and flushed with a reactive gas mixture for an extended time period prior to initiating recrystallization of the melt. This latter process, while relatively effective, required the expenditure of an enormous amount of energy and required an exceedingly large amount of reactant gases to getter OH.sup.- and O.sup.-- contaminants. The bake-out and purge process requires large expenditures of time and it is not amenable to production operations. In addition, in many instances where fluoride crystals were being prepared under an HF atmosphere, the corrosive nature of HF tended to create more problems than it solved. HF has proven to be ineffective in gettering OH.sup.- and O.sup.-- from the highly water-contaminated environment and in precluding the inclusion of said ions in the alkaline earth fluoride crystals.
Fluoride crystals could be prepared under a HF atmosphere using a platinum crucible of the design shown in U.S. Pat. No. 4,110,080 issued to Antonio Pastor. However, the expense of fabricating the crucible renders such a method impractical in a production facility. In addition, platinum forms carbon complexes at the temperatures involved which contaminate the crystals. Attempts to prepare fluoride crystals in a crucible of the "080" Pastor design fabricated from graphite failed when the crucible structural integrity failed. Thus, there is still a need for an apparatus and a process suitable for forming OH.sup.- and O.sup.-- free alkaline earth fluoride single crystals and for efficiently forming other halide crystals which are OH.sup.- and O.sup.-- free. The present invention is therefore the result of efforts intended to satisfy this need.